1. Field of Invention
The present invention relates generally to the braking of an internal combustion engine, specifically to engine braking apparatus and method for converting an internal combustion engine from a normal engine operation to an engine-braking operation.
2. Prior Art
It is well known in the art to employ an internal combustion engine as brake means by, in effect, converting the engine temporarily into a compressor. It is also well known that such conversion may be carried out by cutting off the fuel and opening the exhaust valve(s) at or near the end of the compression stroke of the engine piston. By allowing compressed gas (typically, air) to be released, energy absorbed by the engine to compress the gas during the compression stroke is not returned to the engine piston during the subsequent expansion or “power” stroke, but dissipated through the exhaust and radiator systems of the engine. The net result is an effective braking of the engine.
An engine brake (or engine retarder) is desirable for an internal combustion engine, particularly for a compression ignition type engine, also known as a diesel engine. Such engine offers substantially no braking when it is rotated through the drive shaft by the inertia and mass of a forward moving vehicle. As vehicle design and technology have advanced, its hauling capacity has increased, while at the same time rolling and wind resistances have decreased. Accordingly, there is a heightened braking need for a diesel-powered vehicle. While the normal drum or disc type wheel brakes of the vehicle are capable of absorbing a large amount of energy over a short period of time, their repeated use, for example, when operating in hilly terrain, could cause brake overheating and failure. The use of an engine brake will substantially reduce the use of the wheel brakes, minimize their wear, and obviate the danger of accidents resulting from brake failure.
There are different types of engine brakes. Typically, an engine braking operation is achieved by adding an auxiliary engine valve event called an engine braking event to the normal engine valve event. Depending on how the engine valve event is produced, an engine brake can be defined as:                (a) Type I engine brake—the engine braking event is produced by importing motions from a neighboring cam, which generates the so called “Jake” brake;        (b) Type II engine brake—the engine braking event is produced by altering existing cam profile, which generates a lost motion type engine brake;        (c) Type III engine brake—the engine braking event is produced by using a dedicated valve lifter for engine braking, which generates a dedicated cam (rocker) brake;        (d) Type IV engine brake—the engine braking event is produced by modifying the existing engine valve lift, which normally generates a bleeder type engine brake;        (e) Type V engine brake—the engine braking event is produced by using a dedicated valve train for engine braking, which generates a dedicated valve (the fifth valve) engine brake.        
The engine brake can also be divided into two big categories, i.e., the compression release engine brake (CREB) and the bleeder type engine brake (BTEB).
Compression Release Engine Brake (CREB)
Conventional compression release engine brakes (CREB) open the exhaust valve(s) at or near the end of the compression stroke of the engine piston. They typically include hydraulic circuits for transmitting a mechanical input to the exhaust valve(s) to be opened. Such hydraulic circuits typically include a master piston that is reciprocated in a master piston bore by a mechanical input from the engine, for example, the pivoting motion of the injector rocker arm. Hydraulic fluid in the circuit transmits the master piston motion to a slave piston in the circuit, which in turn, reciprocates in a slave piston bore in response to the flow of hydraulic fluid in the circuit. The slave piston acts either directly or indirectly on the exhaust valve(s) to be opened during the engine braking operation.
An example of a prior art CREB is provided by the disclosure of Cummins, U.S. Pat. No. 3,220,392 (“the '392 patent”), which is hereby incorporated by reference. Engine braking systems based on the '392 patent have enjoyed great commercial success. However, the prior art engine braking system is a bolt-on accessory that fits above the overhead. In order to provide space for mounting the braking system, a spacer may be positioned between the cylinder head and the valve cover that is bolted to the spacer. This arrangement may add unnecessary height, weight, and costs to the engine. Many of the above-noted problems result from viewing the braking system as an accessory to the engine rather than as part of the engine itself.
As the market for compression release-type engine brakes (CREB) has developed and matured, there is a need for design systems that reduce the weight, size and cost of such retarding systems. In addition, the market for compression release engine brakes has moved from the after-market, to original equipment manufacturers. Engine manufacturers have shown an increased willingness to make design modifications to their engines that would increase the performance and reliability and broaden the operating parameters of the compression release-type engine brake.
One possible solution is to use a dedicated valve lifter for the engine braking U.S. Pat. No. 5,626,116 (“the '116 patent”) discloses a dedicated engine braking system (a Type III engine brake) including a rocker arm having a plunger, or braking piston, positioned in a cylinder integrally formed in one end of the rocker arm wherein the plunger can be locked in an outer position by hydraulic pressure to permit braking system operation. A solenoid valve or control valve is also integrated into the dedicated rocker arm. A cam designed exclusively for engine braking has only the small cam lobes for engine braking Therefore, the engine braking performance can be optimized without interfering with the valve lift profile design for the normal engine operation. During the normal engine operation, the control valve sits in a dent on the rocker shaft and the engine braking rocker arm stays in a neutral position. There are one gap between the rocker arm and the cam and another gap between the rocker arm and the valve bridge.
Although the engine brake system disclosed in the '116 patent has enjoyed considerable commercial success due to its high performance and compact size, it has some drawbacks. One of the drawbacks is that the engine braking rocker arm could get away from the neutral position and contact the cam and the valve bridge during the normal engine operation. The braking piston in the rocker arm would be hammered and get loose to cause serious engine damage.
Additional disadvantages of the prior art system reside in their relative complexity and the necessity for using precision components because of the need of accurate control of the rocker arm position and the braking piston stroke. Thus the system is comparatively expensive and difficult or impossible to install on certain engines.
Another integrated engine braking system for commercial vehicles is known from U.S. Pat. No. 6,234,143 (“the '143 patent”) in which an integrated rocker brake with one-valve opening for engine braking is disclosed. An engine brake actuator is disposed in the rocker arm between the pivot point and the distal end. The rocker arm and the valve bridge of the engine are so arranged that the hydraulic piston of the brake actuator is able to actuate on the inner valve near the pivot point of the rocker arm. By actuating only one of the two exhaust valves, the load from engine braking is greatly reduced.
The above integrated engine brake system, however, has the following drawbacks. First, after the braking valve is lifted by the hydraulic piston, the valve bridge is tilted and the followed normal valve actuation on both the braking valve and non-braking valve by the rocker arm is asymmetric or unbalanced. Large side load could be experienced on both valve stems or on the valve bridge guide if the bridge is guided. Second, the brake system can only fit on a particular type of engines that have the “parallel” arrangement of the rocker arm and the valve bridge.
U.S. Pat. No. 6,253,730 (“the '730 patent”) discloses an integrated rocker brake with a reset valve trying to avoid the asymmetric loading on the valves or the valve bridge caused by the engine braking operation as disclosed by the '143 patent. The reset valve will reset or retract the braking piston in the rocker arm before the braking valve reaches its peak braking lift so that the braking valve will return back to its seat before the main valve lift event starts, and the rocker arm can act on the leveled valve bridge and open both the braking valve and the non-braking valve without any asymmetric loading.
However, resetting the engine brake before the peak braking valve lift is very problematic. First, the duration and magnitude of the valve lift for engine braking is very small and even smaller for resetting. Second, the resetting happens at the peak engine braking load and causes high pressure or large load on the reset valve. The timing for the resetting is critical. If the resetting happens too soon, there will be too much braking valve lift loss (lower lift and earlier closing) and lower braking performance. If the resetting happens too late, the braking valve will not be able to close before the main valve event starts and cause asymmetric loading. Therefore, this type of integrated rocker engine brake may not work well at high engine speeds when the reset duration and height is extremely small and the braking load or pressure on the reset valve is very high.
Bleeder Type Engine Brake (BTEB)
The operation of a bleeder type engine brake (BTEB) has also long been known. During bleeder type engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open during a portion of the cycle (partial-cycle bleeder brake) or open continuously throughout the non-exhaust strokes (intake stroke, compression stroke, and expansion or power stroke) (full-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the former does not have exhaust valve lift during most of the intake stroke.
U.S. Pat. No. 5,692,469 and U.S. Pat. No. 7,013,867 (“the '469 and '867 patents”) disclose a bleeder type engine brake (BTEB) system for engines with one and two exhaust valves per cylinder. The BTEB system works with a throttling device (also known as an exhaust brake) capable of raising exhaust pressure high enough to cause each exhaust valve to float near the end of each intake stroke. In this intermediate opening or floating of the exhaust valve, it is possible to intervene with the braking device so that the exhaust valve, which is about to close after the intermediate opening, is intercepted by a control piston charged with oil pressure and prevented from closing to create a partial cycle bleeder braking event. This is a Type IV engine brake.
The BTEB system of the type described above may not be reliable because it depends on the intermediate opening or floating of the braking exhaust valve, which is inconsistent, both in timing and magnitude. As is well known in the art, exhaust valve floating is highly engine speed dependent and affected by the quality and control of the exhaust brake, and also the design of the exhaust manifold. There may be not enough or none valve floating for the actuation of the engine braking device at middle and low engine speeds when the engine brake is highly demanded since the engine is mostly driving at such speeds. It is clear from the above description that the prior-art engine brake systems have one or more of the following drawbacks:                (a) The system is difficult to stay at a neutral position and could cause engine damage;        (b) The system is difficult to manufacture and has high complexity and cost;        (c) The system is not reliable and only work at certain engine speeds; and        (d) The system has unbalanced load on engine valves.        